1. Field of the Invention
The present invention relates to a transport network, and more particularly to an asynchronous transport network for transmitting higher-layer protocol data.
2. Description of the Related Art
In recent years, technology has been come into practical use for encapsulating data of a higher-layer protocol (hereinbelow, referred to as “higher-layer protocol data”) such as Media Access Control (MAC) frames on Ethernet and transmitting the data on a transport network. This type of technology corresponds to, for example, PPP over SONET (see IETF RFC 2615 Specifications, June 1999, IETF) or GFP (Generic Framing Procedure: See ITU-T G. 7041 Specifications, December 2001, ITU-T).
However, the above-described prior art has a number of problems. For example, the transmission of higher-layer protocol data without any loss on a transport network that is realized by the GFP or PPP-over-SONET technology requires that the transmission devices that make up the asynchronous transport network have both the capability for this transmission and a great processing capacity.
In a transmission device that transfers a signal that has been received from one device to another device in an asynchronous transport network, the speed of the received signal is not necessarily equal to the speed of the transmitted signal. The transmission device must therefore be provided with means for converting the signal speed.
For example, when transferring a signal (GFP frames) in which higher-layer protocol data have been encapsulated by GFP technology, the transmission device adjusts the average signal speed by expanding and shrinking the space between GFP frames. The transmission device similarly adjusts the signal speed when transferring a signal (HDLC-like frames (refer to IETF RFC 1662 specifications)) that has been encapsulated by PPP-over-SONET technology.
In both GFP and PPP, however, the functions of the protocol are not capable of interrupting the transfer of frames and then subsequently restarting. In order to adjust the spacing between frames, a transmission device must therefore recognize the boundary between frames, temporarily store for each frame, and then transmit at appropriate timing.
In other words, the transmission device must be provided with the capability of detecting the boundaries of frames from the received signal. In the case of GFP, for example, the transmission device requires a circuit for establishing frame synchronization, and the circuit scale is accordingly increased.
The transmission device further requires a frame buffer for storing frames. In particular, when handling the so-called jumbo frames having a length that exceeds 8 kilobytes, the transmission device requires a large frame buffer having a large memory capacity that can store jumbo frames. Temporarily storing such a long frame also entails an increase in the frame processing delay in the transmission device.